1. Field of the Invention
The present invention relates to a two-photon absorption material, in particular, a two-photon absorption material having a large two-photon absorption cross-sectional area, which is applied to, for example, materials for various organic electronics devices such as three-dimensional memory materials, light restricting materials, photocurable resins for optical molding (curable materials), materials for photo-chemotherapy, and fluorescent dyes for two-photon fluorescence microscopy.
2. Description of the Related Art
Two-photon absorption is a type of three-dimensional nonlinear optical effects. This is a phenomenon in which one molecule absorbs two photons simultaneously to transit from the ground state to the excited state. The two-photon absorption has been known for a long period of time. In recent years, studies on materials having two-photon absorptivity have been advanced, since Jean-Luc Bredas et al. unraveled the relationship between the molecular structure and the mechanism (Science, 281, 1653 (1998)).
Two-photon absorption materials can be excited with light having a wavelength falling within a non-resonant region. The actual excited state thereof is at an energy level about twice as much as that of photons used for the excitation. The efficiency of transition caused by two-photon absorption is proportional to the square of the intensity of an optical field applied (square-law characteristics of two-photon absorption).
Thus, when a laser beam is applied, two-photon absorption occurs only at a position where the electric field intensity is high; i.e., the center of a laser spot, and two-photon absorption does not occur at all at a position where the electric field intensity is low; i.e., the peripheral portion.
Hitherto, Japanese Patent Application Laid-Open (JP-A) Nos. 2005-213434, 2005-82507, 2004-168690, 2007-178585 and other patent literatures disclose two-photon absorption materials.
Also, JP-A Nos. 2005-500394, 2004-168690, 2004-339435, 2005-263738, 2006-178399, 2006-209059, 2006-251351, 2007-91684, 2007-119443 and other patent literatures disclose porphyrin derivatives serving as two-photon absorption materials.
In a three-dimensional space, two-photon absorption occurs only at an area where the electric field intensity is high; i.e., a focal point of the laser beam condensed by a lens. Meanwhile, two-photon absorption does not occur at all at an area where the electric field intensity is low; i.e., an area other than the focal point.
Compared with one-photon linear absorption in which excitation occurs at all the positions proportionally to the intensity of the optical field applied, for two-photon absorption, excitation occurs only at a pinpoint region in the space by virtue of the square-law characteristics. Thus, spatial resolution is remarkably improved.
By utilizing the above characteristics, three-dimensional memory has been studied which records bit data by changing spectrum, refractive index, or polarized light at predetermined positions of recording media. And, attempts have been made to apply two-photon absorption materials to three-dimensional multi-layer memory.
For example, networks (e.g., the Internet) and high-vision TVs become popular rapidly, and high-definition television (HDTV) broadcast is starting soon. Thus, demand for mass-storage recording media has been increasing, in order to record image information of 50 GB or higher (preferably 100 GB or higher) inexpensively and simply in consumer use.
Hitherto, JP-A Nos. 2005-100606, 2005-517769, 2004-534849 and other patent literatures disclose such three-dimensional memory media (materials).
Also, JP-A No. 08-320422 and other patent literatures disclose a light restricting device (material).
Also, JP-A No. 2005-134873 and other patent literatures disclose optical molding.
Also, JP-A Nos. 09-230246, 10-142507, 2005-165212 and other patent literatures disclose (fluorescent) microscopes utilizing two-photon absorption.
Separately, the present inventors have previously developed and proposed techniques relating to two-photon absorption materials (see JP-A Nos. 2007-246463, 2007-246790, 2008-69294, 2008-214303, 2007-241170, 2007-246422 and 2008-163184).
Furthermore, optical recording media have been demanded for business applications (e.g., computer and broadcast backups) in order to inexpensively record mass information of about 1 TB or higher at high speed. Under such circumstances, the capacity of conventional two-dimensional optical recording media (e.g., CDs and DVDs) is at most about 25 GB on physical principles, even if a recording/reproducing wavelength is shortened. This capacity is not enough considering use thereof in future.
In view of this, three-dimensional optical recording media have been prominently expected as ultimate high-density, high-capacity recording media.
In the three-dimensional optical recording media, information can be recorded in the form of several tens or several hundreds of superposed layers in a three-dimensional (thickness) direction to achieve ultra high-density, ultra high-capacity recording which is several ten or several hundred times higher than conventional two-dimensional recording media.
To provide three-dimensional optical recording media, information needs to be written with access to a predetermined area in a three-dimensional (thickness) direction. To achieve this, a two-photon absorption material or holography (interference) may be employed.
In three-dimensional optical recording media using a two-photon absorption material, so-called bit recording, which can record information several ten or several hundred times as much as conventional recording, can be attained based on the aforementioned physical principles, thereby achieving higher-density recording. Thus, the three-dimensional optical recording media are ultimately high-density, high-capacity optical recording media.
Regarding three-dimensional optical recording media using a two-photon absorption material, there have been proposed a method of reading information through fluorescence by using a fluorescent compound during recording/reproducing (JP-A Nos. 2001-524245 and 2000-512061); a method of reading information through absorption or fluorescence by using a photochromic compound (JP-A Nos. 2001-522119 and 2001-508221); and other methods. However, any of these literatures do not specifically but abstractly describe two-photon absorption compounds such as two-photon absorption compounds with extremely small two-photon absorption efficiency.
Moreover, the photochromic compounds described in these patent literatures are reversible materials and thus, there are problems in nondestructive readout, long-term archivability of recorded information, an S/N ratio concerning reproduction, and the like. Thus, the photochromic compounds are not suitably used for optical recording media in practical use. In terms of, among others, nondestructive readout and long-term archivability of recorded information, irreversible materials are preferably used since reproduction can be performed based on change in reflectivity (indices of refraction and absorptivity) or luminescence intensity. However, none of them specifically discloses two-photon absorption materials having such properties.
Also, JP-A Nos. 06-28672 and 06-118306 disclose three-dimensional recording apparatuses, reproducing apparatuses therefor, readout methods therefor, and the like, but do not describe a method using a two-photon absorption, three-dimensional optical recording material.
Two-photon absorption can be applied to various applications requiring very high spatial resolution. But, two-photon absorption compounds available at the present time have low two-photon absorptivity and thus, requires expensive, extremely high-power laser as a light source used for causing two-photon absorption. Therefore, in order to employ a small-scale, inexpensive laser to realize practical uses based on two-photon absorption, it is required to develop highly sensitive two-photon absorption materials.
In the above-described porphyrin-based two-photon absorption materials, two-photon absorptivity is imparted to them, for example, as follows. Specifically, a specific color-developing moiety is introduced into a porphyrin skeleton (JP-A No. 2005-500394), porphyrin skeletons are linked to each other (JP-A No. 2004-168690), porphyrin skeletons are condensated with each other (JP-A Nos. 2005-263738 and 2006-178399), and a porphyrin skeleton is elongated (JP-A No. 2006-209059). However, any of the formed skeletons have complicated structures, making it difficult to produce compounds of interest on an industrial scale. In addition, they exhibit an insufficiently large two-photon absorption cross-sectional area.
Also, the present inventors have found that a specific azaporphyrin, in which a carbon atom at a meso position of the porphyrin skeleton has been substituted with a nitrogen atom, exhibits excellent two-photon absorptivity, and have previously disclosed it in JP-A No. 2008-163184. But, this compound does not exhibit a sufficiently large two-photon absorption cross-sectional area.
As described above, conventional compounds have a small two-photon absorption cross-sectional area, which indicates two-photon absorptivity per molecule. Particularly when a femtosecond pulse laser is used, most of the compounds exhibit a two-photon absorption cross-sectional area smaller than 200 GM (1 GM=1×10−50 cm4·s·molecule−1·photon−1). Thus, industrial applications of these compounds have not been made yet.
As described above, two-photon absorption can be applied to various applications requiring very high spatial resolution. But, two-photon absorption compounds available at the present time have low two-photon absorptivity and thus, requires expensive, extremely high-power laser as a light source used for causing two-photon absorption.
Therefore, in order to employ a small-scale, inexpensive laser to realize practical uses based on two-photon absorption, it is required to develop highly sensitive two-photon absorption materials.
But, two-photon absorption compounds available at the present time have low two-photon absorptivity. Thus, it is necessary to use an extremely high-power laser as a light source, and it takes a long period of time for information recording.
In particular, in use thereof in three-dimensional optical recording media, in order to achieve a high transfer rate, it is necessary to design a two-photon absorption, three-dimensional optical recording material which attains recording with high sensitivity based on the differences in optical characteristics (e.g., luminescence and reflection) through two-photon absorption. To achieve this, a material containing a two-photon absorption compound and a recording component is effective, wherein the two-photon absorption compound can be at an excited state by absorbing two photons with high efficiency, and the recording component can efficiently form the differences of optical characteristics (e.g., luminescence and reflection) of the two-photon absorption optical recording material by a method utilizing the excited state of the two-photon absorption compound. But, such a material has been hardly disclosed, and demand has arisen for the material and the method for designing it.